MOSFET (metal oxide semiconductor field effect transistor) is a semiconductor device used for applications such as power control. There has been proposed a structure in which a diode is built in such a MOSFET. In the operation of the MOSFET, the drain electrode is positively biased relative to the source electrode. Thus, the current flows from the drain electrode to the source electrode. On the other hand, in the operation of the diode, the drain electrode is negatively biased relative to the source electrode. Thus, the built-in diode is turned on, and the current flows from the source electrode to the drain electrode.
In a typical MOSFET used for applications such as power control, the p-base region in contact with the source electrode serves as a p-region (anode) of the p-n diode. The n-region in contact with the drain electrode serves as an n-region (cathode) of the p-n diode. Thus, the MOSFET includes a p-n diode therein. When the drain is negatively biased at gate-off, holes flow from the p-base region in contact with the source electrode, and electrons flow from the n-region in contact with the drain electrode. Thus, the built-in diode is turned on.
However, the injected holes may compromise the reliability of the gate oxide film and the semiconductor material itself of the MOSFET. For instance, when SIC is used for the semiconductor material, the base plane dislocation part in the SiC semiconductor layer is supplied with energy generated by recombination of the injected holes and electrons. Thus, the base plane dislocation is transformed to a stacking fault. This causes a problem that the SiC semiconductor layer is turned to a high resistance layer.
Thus, in a MOSFET used for applications such as power control, there is demand for a structure suppressing hole injection at the time of operation of the built-in diode to improve reliability.